1. Field of the Invention
This invention relates to a color-developing sheet for pressure-sensitive recording sheets using as a color-developing agent an inorganic solid acid, characterized in that the color-developing sheet has an improved preservability.
2. Prior Art
Pressure-sensitive recording sheets are known as carbonless copying paper. They produce a color upon the application of a mechanical or impact pressure by writing or by pounding a typewriter, thus permitting duplication of several copies. A colored image is formed by a color-developing reaction of a electron-donating colorless dyestuff and a electron-accepting color-developing agent.
Hitherto, many kinds of electron-accepting color-developing agents are well-known. The typical color-developing agents include inorganic solid acids such as acid clay, activated clay, attapulgite (described in U.S. Pat. No. 2,712,507); substituted phenols and diphenols (described in the Japanese Patent Publication No. 9309/1985); p-substituted phenol-formaldehyde polymers (described in the Japanese Patent Publication No. 20144/1967); aromatic carboxylic acid metal salts (described in the Japanese Patent Publication Nos. 10856/1974 and 1327/1977); 2,2'-bisphenol sulfone compounds (described in the Japanese Patent Laid-Open No. 106313/1979); and so on.
An inorganic solid acid adsorbes the above electron-donating colorless dyestuff and is reacted with it to form a colored image. As an inorganic solid acid, there are included acid clay as montmorillonite type clay; activates clay which is prepared by treating an acid clay with a mineral acid; attapulgite which is a magnesium-aluminum-silicate mineral; and the like.
These inorganic solid acids have been widely applied for practical use as color-developing agent for pressure-sensitive recording sheet, since they provide a higher color-developing rate, a superior image density and a clear color tone owing to a superior adsorption ability. However, they have the defects that a colored image fades easily under exposure of ultraviolet ray, etc., a black-developed image discolor gradually to reddish black, a colored image disappears for some time in presence of water, and the like. In order to overcome these defects, there have been proposed an improved process for producing an inorganic solid acid and its use in combination with a polyvalent metal salt, an aromatic carboxylic acid, or the like. However, these proposed methods provide yet no sufficient results. As described above, a color-developing sheet containing an inorganic solid acid has defects that sunlight fading-resistance and water-resistance are inferior.